Transparent conductive oxide films have high transmittance in the visible region and high conductivity, and are used in electrodes of liquid crystal display units or various light receiving elements such as solar cells. They are also widely used in thermic ray reflective coatings and antistatic films for automobile and construction materials, and anti-fogging transparent heating units in freezer showcases and the like. One such transparent conductive oxide film that is used is a zinc oxide-based film having an element such as aluminum, gallium or boron added to zinc oxide. Aluminum oxide-added zinc oxide-based films, in particular, have excellent light transmittance in the infrared region, and are therefore suited for purposes that are highly dependent on optical transparency, such as solar cells.
Film-forming methods used for such transparent conductive oxide films include sputtering methods employing sputtering targets, because they allow formation of films with uniform film thicknesses over large areas. However, such sputtering methods have been associated with problems, including a reduced operating rate of the sputtering apparatus due to anomalous discharge that occurs during sputtering, and lower product yield due to the effect of generated particles.
As means for minimizing anomalous discharge produced during sputtering, PTL 1 discloses a ZnO-based sintered material containing 3 to 7 atomic percent Al, and 0.3 to 3 atomic percent of at least one third element selected from the group consisting of B, Ga, In, Ge, Si, Sn and Ti. However, with the compositional control described in PTL 1, it is not possible to sufficiently minimize anomalous discharge during sputtering. It is therefore desirable to even further reduce anomalous discharge.
A film obtained using a sputtering target must exhibit not only the properties of low resistance and high transmittance in a wide wavelength range, and especially not only in the visible region but also in the infrared region, and high stability (durability) of the film properties. Based on experimentation by the present inventors, however, films obtained using the sputtering target of PTL 1 do not readily exhibit both high optical transparency in a wide wavelength range, and especially in the infrared region, and high stability (durability) of the film properties.
Also, while in terms of optical transparency, transmittance is exhibited at specific wavelengths of 550 nm and 1000 nm, for practical use it is important for high transmittance to be obtained not only at specific wavelengths but also across the entire wavelength range demanded for the purpose of use. When durability has been increased with the film of PTL 1, it has been found that the transmittance is reduced in the long wavelength range of longer than the wavelength of 1000 nm mentioned in the examples. It is desirable to ameliorate this phenomenon for purposes that depend on optical transparency, such as a solar cell.
PTL 2 discloses a ZnO-based sintered material that (1) has a major compositional phase which is a ZnO phase with solid solution of 0.2-14 atomic percent of at least one element selected from the group consisting of B, In, Al, Ga, Ge, Sn, Si and Ti, (2) has a sintered density of 4.5 g/cm3 or greater, (3) has a volume resistivity of no greater than 1 kΩcm and (4) has a mean crystal grain size of 2-20 μm. However, PTL 2 does not contain examples of adding Al and Ti in combination, nor does it contain any mention of the composition or properties.
PTL 3 discloses a zinc oxide sintered material containing at least one element from among Al, Ga, In, Ti, Si, Ge and Sn added to ZnO. The zinc oxide sintered material has precipitates each containing a composite oxide phase of the added element and zinc, and voids formed surrounding the precipitates. There is also disclosed a zinc oxide sintered material wherein, of the precipitates, the proportion of precipitates with a circle equivalent diameter of at least 3 μm is no greater than 20%, and of the voids, the proportion of voids with a circle equivalent diameter of at least 3 μm is no greater than 50%.
Further disclosed is a zinc oxide sintered material containing, with ZnO, one first added element from among Al, Ga, In, Ti, Si, Ge and Sn, and a second added element which is at least one element from among Al, Ga, In, Ti, Si, Ge and Sn, which has not been added as the first added element. The zinc oxide sintered material is disclosed as a zinc oxide sintered material having a co-present zone wherein a first precipitate comprising a composite oxide phase of the first added element and zinc and a second precipitate comprising a composite oxide phase of the second added element and zinc are co-present in a zinc oxide phase as the main phase.
However, it is not possible to sufficiently minimize anomalous discharge during sputtering simply by controlling the microstructure in the sintered material (the compositional phase and its particle sizes and void diameters), as disclosed in PTL 3. Furthermore, it is impossible to avoid problems such as decreased yield due to particles that fly off during sputtering, and the resulting loss of productivity. It is therefore desirable to even further reduce anomalous discharge.
On the other hand, PTL 4 has disclosed a zinc oxide-based transparent conductive film containing Al and Ti, as a thin-film containing such elements. However, PTL 4 is based on a chip-on film forming method that employs DC magnetron sputtering. In this process, a portion of the element composing the thin-film is formed as a chip and placed on the target for sputtering. Hence, it contains no disclosure regarding the sintered material (sputtering target).
In light of the goal of improving light transmittance in the infrared region, it is also known that zinc oxide films with reduced aluminum oxide addition and zinc oxide films containing no aluminum oxide as added material, provide highly superior light transmittance in the infrared region. Their durability has been poor, however, and it has been difficult to achieve both high transmittance in the infrared region and high durability.